The goal of the program is to clarify the relations between events in ontogeny and the development, maintenance, and reversal of dyadic behaviors associated with aggression. Toward this end, the work will focus upon the mechanisms by which prior experiences serve to influence reactivity to others. The long-range objective of this work is to contribute to the establishment of empirical and theoretical generalizations on aggressive behavior development in mammals. Accordingly, detailed accounts of the processes of development in selected species are required to determine which mechanisms are relatively specific and which are general. The species selected for the present ontogenetic analysis are selectively bred and controlled mouse strains. Observational, experimental, and genetic procedures will be employed in the research. An interactional coding system for the analysis of reciprocal social behavior patterns will be applied to both naturalistic and experimental settings. One of the principal tasks of the work will be done to determine the generality of the developmental and evolutionary principles (i.e., developmental plasticity, behavioral neotony) that have been identified in earlier studies. The research will continue to focus on the issue of how the effects of prior social interactions can be modified and reversed at maturity. Our direct observations will be augmented an extended by the use of advance procedures for the audio-visual recording of the primary behaviors. In summary, the progarm will attempt to provide a coherent account of the mechanisms through which developmental factors promote, inhibit, or maintain aggressive interactions.